You may safely use any libgit2 object from any thread, though there may be issues depending on the cryptographic libraries libgit2 or its dependencies link to (more on this later). For libgit2 itself, provided you take the following into consideration you won't run into issues:
Use an object from a single thread at a time. Most data structures do not guard against concurrent access themselves. This is because they are rarely used in isolation and it makes more sense to synchronize access via a larger lock or similar mechanism.
There are some objects which are read-only/immutable and are thus safe to share across threads, such as references and configuration snapshots.
The error message is thread-local. The
giterr_last() call must happen on the same thread as the error in order to get the message. Often this will be the case regardless, but if you use something like the GCD on Mac OS X (where code is executed on an arbitrary thread), the code must make sure to retrieve the error code on the thread where the error happened.
When built as a native Windows DLL, libgit2 uses WinCNG and WinHTTP, both of which are thread-safe. You do not need to do anything special.
When using libssh2 which itself uses WinCNG, there are no special steps necessary. If you are using a MinGW or similar environment where libssh2 uses OpenSSL or libgcrypt, then the general case affects you.
By default we use libcurl to perform the encryption. The system-provided libcurl uses SecureTransport, so no special steps are necessary. If you link against another libcurl (e.g. from homebrew) refer to the general case.
If the option to use libcurl was deactivated, the library makes use of CommonCrypto and SecureTransport for cryptographic support. These are thread-safe and you do not need to do anything special.
Note that libssh2 may still use OpenSSL itself. In that case, the general case still affects you if you use ssh.
By default we use libcurl, which has its own .
If libcurl was not found or was disabled, libgit2 uses OpenSSL to be able to use HTTPS as a transport. This library is made to be thread-implementation agnostic, and the users of the library must set which locking function it should use. This means that libgit2 cannot know what to set as the user of libgit2 may use OpenSSL independently and the locking settings must survive libgit2 shutting down.
libgit2 does provide a last-resort convenience function
git_openssl_set_locking() (available in
sys/openssl.h) to use the platform-native mutex mechanisms to perform the locking, which you may rely on if you do not want to use OpenSSL outside of libgit2, or you know that libgit2 will outlive the rest of the operations. It is not safe to use OpenSSL multi-threaded after libgit2's shutdown function has been called.
If your programming language offers a package/bindings for OpenSSL, you should very strongly prefer to use that in order to set up locking, as they provide a level of coördination which is impossible when using this function.
See the OpenSSL documentation on threading for more details.
Be also aware that libgit2 does not always link against OpenSSL if there are alternatives provided by the system.
libssh2 may be linked against OpenSSL or libgcrypt. If it uses OpenSSL, you only need to set up threading for OpenSSL once and the above paragraphs are enough. If it uses libgcrypt, then you need to set up its locking before using it multi-threaded. libgit2 has no direct connection to libgcrypt and thus has not convenience functions for it (but libgcrypt has macros). Read libgcrypt's threading documentation for more information
It is your responsibility as an application author or packager to know what your dependencies are linked against and to take the appropriate steps to ensure the cryptographic libraries are thread-safe. We agree that this situation is far from ideal but at this time it is something the application authors need to deal with.